The domain you're sending mail to doesn't have our mailservers listed as the primary MX record.

The DNS records for the domain you're sending mail to have recently been updated.

There is a forwarder on the receiving end of the e-mail address you are sending to that forwards to another e-mail address that forwards back to the initial e-mail address, creating an infinite loop.

This isn't a comprehensive list, but these are the most common reasons for seeing this error. Read below to see how to address each issue.

Solutions

Here is the list of solutions you'll need in the event of seeing this error and one of the above reasons being the cause:

In order to send mail through our mail servers, your email client must be configured to use SMTP Authentication. That is, our mail servers needs to be logged into with a username and password.Common email clients include a tickbox in the account configuration with the label "This server requires authentication". That box should be ticked, otherwise look for a setting on the outgoing server settings that refers to authentication and set it to use the same username and password for the incoming server.If there's not an option to use the same credentials as the incoming mail server, you should set your full e-mail address as the username, and then add your mailbox password to the password field.

Some ISPs, such as AOL, block sending mail on port 25. If this is the case for your ISP, setting SMTP to port 587 will in almost all circumstances be a working alternative.

Our mailservers won't accept mail for domains that don't have our mail servers as the primary MX records (i.e. the records with the lowest priority number set). If you get a 550 error when sending to our servers, and they aren't the primary records, you'll need to raise the priority of the MX records that point to our mail servers by setting them with the lowest number of the existing records.

You may also see this error shortly after a DNS change has been made. If a domain's DNS cache has not yet updated to reflect recent changes, you may face 550 errors. Waiting will resolve the matter in this instance.

This is simply a case of removing the forwarder that completes the loop. Remember that you can set forwarders on auto-responders, so if this forwards back to the e-mail address the auto-responder is for, this will need to be changed or removed.

If you continue to receive the error, please contact support, they will be able to assist you further.